This lecture covers the principles and operation modes of gaseous ionisation detectors, including ionisation chambers, proportional counters, and Geiger-Müller counters. It explains the detection regions, operation modes, and applications of each type of detector, highlighting their advantages and disadvantages. The lecture also discusses the avalanche formation in these detectors, the choice of geometry, and the properties of proportional counters and Geiger-Müller counters. It concludes by comparing the main properties and typical uses of ionisation chambers, proportional counters, and Geiger-Müller counters in radiation detection.